1. Field of the Invention
This invention relates generally to a feed system for an antenna, and, more particularly, to a feed system used in connection with a low observable, multifunction, conformal, load bearing structure antenna on an aircraft that provides an RF feed for VHF/FM, VHF/AM, UHF and L-band frequencies.
2. Discussion of the Related Art
Modern manned and unmanned tactical military aircraft require radio communications over several frequency bandwidths and communication modes to support the communications, navigation and identification (CNI) functions necessary for operation of the aircraft. These radio frequency (RF) bandwidths generally include the VHF frequency modulation (FM) band (30-88 MHz), the VHF amplitude modulation (AM) band (118-174 MHz), UHF band (225-400 MHz), and L BAND (962-1213 MHz). The required L-band functions are JTIDS, MIDS, and TACAN.
To transmit and receive signals in these frequency bands, a suitable antenna system is required that is configured on the aircraft. Generally, multiple blade antennas are required for the CNI functions, including one for the VHF/FM frequency band, one for the VHF/AM frequency band and another one for the UHF frequency band. the available antenna installation sites on an aircraft may not support the number of antennas needed by the required CNI functions if each function requires its own antenna. Because modern tactical aircraft are usually low-observable aircraft, it is necessary that the antenna elements conform to the aircraft structure to minimize the radar cross-section (RCS) of the aircraft and that require a feed that connects separate conductive structures.
To overcome the requirements of multiple antennas to support the CNI functions, and to eliminate the need for blade antennas, U.S. Pat. No. 5,825,332 issued Oct. 20, 1998, titled "Multifunction Structurally Integrated VHF-UHF Aircraft Antenna System," assigned to the assignee of this application and herein incorporated by reference, discloses an aircraft antenna that is totally integrated within the aircraft, and operates over a wide range of frequencies, including the VHF/FM, VHF/AM and UHF frequency bands. To operate in this manner, the antenna system uses an electrically conductive element that is part of the aircraft structure and an antenna element positioned and shaped to form a notch therebetween. The notch is generally uniform in width over part of its length and flares to a larger width over the remainder of its length. Broadband impedance matching electronics are provided to couple the antenna system to a transceiver to provide efficient transfer of energy to and from the antenna.
Other antenna systems have also been proposed that conforn with the existing aircraft structure. For example, U.S. patent application Ser. No. 09/178,356, filed Oct. 23, 1998, titled "A Conformal Load-Bearing Antenna System," assigned to the assignee of this application, and herein incorporated by reference, discloses a conformal load bearing antenna structure (CLAS) that is configured within the skin of the aircraft so that the antenna element does not increase the RCS of the aircraft. The CLAS manufacturing processes allow the antenna elements to be integrated within a composite RF window that carries a load that would be carried by the replaced skin panel. The CLAS antenna elements also use part of the aircraft skin as the radiating element.
Antenna elements manufactured with the CLAS manufacturing process require a suitable RF feed that connects the antenna element to the matching electronics. U.S. patent application Ser. No. 09/233,361, titled "A Dual-Feed System for a Multifunction, Conformal, Load-Bearing Structure Excitation Antenna," filed Jan. 19, 1999, assigned to the assignee of this application, and herein incorporated by reference, discloses a dual-feed system for providing a feed structure within the notch between the antenna element and the conductive aircraft structure of the type discussed in the '332 patent. The feed structure includes two feed posts strategically positioned along the notch so that the feed posts provide impedance matching at the desirable frequencies. In one particular design, an aft feed post provides impedance matching at a feed point location for the UHF and VHF/FM bands, and a forward feed post provides impedance matching at a feed point location for VHF/AM frequencies. The feed posts can be made of any suitable conductive material, such as brass, gold, nickel, etc., and can have any suitable shape, such as cylindrical and conical shapes. The feed posts are connected to the antenna element and the aircraft structure by a solder connection.
The aircraft structure is subject to high vibration and temperatures during operation. Therefore, the feed connection must be able to withstand this environment. The CLAS antenna elements are actual load bearing parts of the aircraft, such as a vertical tail end cap or fin cap. Feed structures with a soldered or rigid mechanical connection would quickly fail. Additionally, it is necessary to provide a feed connection that provides impedance matching for all of the VHF/AM, VHF/FM, UHF and L-band frequencies.
What is needed is a compliant RF feed connection that satisfies the above requirements. It is therefore an object of the present invention to provide such an RF feed.